What is the ruling on a small drop of urine exiting after performing ablution (wudu') or during prayer (salat)? And is it obligatory to repeat the ablution or to interrupt the prayer and change one's clothes?

If you are certain that a drop of urine came out after ablution, your ablution is nullified and must be redone. If you are certain that it came out during prayer, both your ablution and prayer are invalidated. However, if it is merely a matter of waswasah (whispers from Satan/obsessive thoughts), then your prayer and ablution are valid, for the default state is that ablution is not nullified unless its nullification is confirmed. Prayer is valid with a small drop (of impurity) on the garment, according to some scholars.